In industrial production, quality control has become an increasingly important part of the manufacturing process. As the products get complicated, the tolerances are reduced and manufacturing lots increased, various machine vision systems have proven good due to their speed and the measurement manner non-contacting the article.
However, the number of problems that are suited to be solved using machine vision systems often is restricted. Typically, machine vision can be used to carry out separate checking and measurement operations two-dimensionally. In case there are more than one object being measured, each measurement is usually made using separate imaging devices and even at different measuring stations or manufacturing cells. In that case, there is no way of finding out the mutual relations and measurements between the details of the entire article. A typical example of a situation such as this represents an article the bores drilled into which shall be located, with sufficient accuracy, in the same straight line for the purpose of installing the shaft at a later point. In that case, each of the bores can be within the tolerances as regards, for example, their dimensions and roundness, but nevertheless the shaft cannot be installed due to the erroneous mutual locations of the bores.
The problem can usually be solved using three-dimensional machine vision technology. Known in prior art is, for example, an arrangement as disclosed in patent FI111755 using which the aforementioned three-dimensional measurement can be implemented.
A conventional three-dimensional measurement system has the limitation that while it allows accurate measurement of individual spots, determining the location and position of a single component attached to the whole may still prove problematic. Typically, this kind of situation occurs when one cannot obtain a sufficient number of accurate observations regarding the component, for example, by measuring a set of known spots. The component does not necessarily contain so many visible spot-like details or details that could be distinguished from one another so that all the degrees of freedom related to the location of the component could be determined. A conventional component of this kind is a nut, a threaded sleeve or a cylinder welded to the hole outside thereof. The machine vision system can be used to measure the diameter and location of the hole, but the welded part does not offer such details to be viewed by the camera that could be used to determine the direction or location of the axis thereof. The wrong position or place of the component will guide the bolt or shaft to be mounted thereto in the wrong direction. At its worst, this will prevent the assembly of the end product. Measuring the place of a component optically from the mounting side is not often possible because of the solid casing structure. Besides, the accurate location of the component, based on the measurement values given by the exterior, does not guarantee the correct position or place of the cylindrical hole or thread included in it, which in the end will, however, decide whether the installation of the shaft or bolt can be successfully implemented.
A conventional way of ensuring the place of a component and the direction of the axis thereof is to use a mechanical gauge, possibly attached to a special measurement jig. In case the gauge fits the hole, the article will do. If, in turn, the gauge does not fit in, the article must be discarded. This method is, however, a very laborious and inflexible one especially when products are manufactured in very big series. A gauge is slow to work with and is a mechanically wearing part, and gauges are usually needed several to secure one object (upper and lower tolerance gauges). A modern production model that is based on statistical quality control methods requires, in addition, collecting of more versatile measurement data on the manufacturing process than the rough “works or not” estimate a gauge can give.
Reference publication US 2002/0029127 A1 discloses an arrangement in which the location of the middle axis of a cylindrical or revolution-shaped article is measured using a stereo camera system. This kind of system involves, however, serious disadvantages, which impede its utilisation in industrial use. The arrangement of the reference publication uses two cameras to produce a stereo image, and the number of cameras cannot be increased to improve the accuracy. Further, the system of the reference publication is based on conventional technology in which the shapes of the articles are determined based on the border lines imaged. Border lines are, however, not easily visible if the object being measured is e.g. a drilled bore; instead in that case the measurement is limited to the mouth of bore while the actual geometry of the bore remains unknown. Industrial applications, however, often use holes that can be straight or slanting, and can be provided with a thread, so the measurement of the hole geometry is desirable. In addition, we must note that the arrangement of the reference publication uses an approximate mathematical model and additional mathematical conversions that surely weaken the measurement accuracy.